1. Field of the Invention
This invention pertains to multi-stage multiple cylinder assemblies and more particularly to seals at the axially inner ends of said cylinders for preventing leakage of higher speed fluid to lower speed cylinders.
2. Description of the Prior Art
Typically, variable speed multiple cylinder assemblies are not machined along the entire length of a cylinder because sealing is required only when the next outer cylinder is fully retracted. That is, when a diametrically inner or higher speed stage is alone energized, the actuating fluid must be sealed from the cylinder of the next lower speed diametrically outer stage to get full utilization of the fluid pressure to the higher speed stage. Thus seals are provided only at the axially inner ends of the cylinders.
It is especially difficult to machine the full length of a cylinder in long cylinders such as used in hydraulic drilling rigs in which the cylinder length may be 40 feet long. A typical hydraulic drilling rig variable speed multi-cylinder assembly is shown in U.S. Pat. No. 3,282,357, the details of which are incorporated herein by reference thereto. The sealing as is shown in this patent is provided by an axially inner packing gland which is engaged by a machined extension of an inner ram for merely a short length of the inner ram. Sealing in this manner is unsatisfactory because the axially inner end of the ram and thus the machined sealing extension is not and cannot be accurately aligned relative to the packing gland such that tolerances must be provided between the packing gland and the machined extension to allow out of alignment conditions. The tolerances create leakage, allow wear and eventually require considerable maintenance or replacement of the packing which tends to extrude out because of the loose fit.